Precoding and feedback channel information in wireless communication system

ABSTRACT

The present invention relates to precoding and feedback channel information in wireless communication system. A method includes receiving a first Precoding Matrix Index (PMI) and a second PMI from a terminal; mapping one or two codewords into layers; precoding symbols mapped into the layers using a first precoding matrix derived from the first PMI and a second precoding matrix derived from the second PMI; and transmitting the precoded symbols to the terminal, wherein the reception of the first PMI is less frequent than the reception of the second PMI.

CROSS-REFERENCE TO RELATED APPLICATION

This Application is a continuation of U.S. patent application Ser. No.13/500,288, filed on Apr. 4, 2012, which is the National Stage Entry ofInternational Application PCT/KR2009/005705, filed on Oct. 6, 2009, allof which are incorporated herein by reference for all purposes as iffully set forth herein.

BACKGROUND Field

The present invention relates to precoding and feedback channelinformation in wireless communication system.

Discussion of the Background

There are a number of multi-antenna transmission schemes or transmissionsuch as transit diversity, closed-loop spatial multiplexing or open-loopspatial multiplexing. Closed-loop MIMO (CL-MIMO) relies on moreextensive feedback from the mobile terminal.

SUMMARY

In accordance with an aspect, there is provided a method or a system,comprising: mapping one or two codewords to the layers; precoding amapped set of symbols using a precoding matrix derived from at least twodownlink channel information where one of them is for rank adaptationand power allocation and the other of them is for the precoding withoutrank adaptation and power allocation and transmitting a signal thatcomprises the precoded set of symbols.

In accordance with another aspect, there is provided a method or asystem for feedbacking channel information for the mobile terminal, themethod comprising: estimating a downlink channel from the receivedsignal; selecting one matrix for rank adaptation plus power allocationand the other matrix for the precoding without rank adaptation and powerallocation based on the estimated channel state information andfeedbacking the PMI (Precoding Matrix Index) of the selected matrix forrank adaptation and power allocation by long term and the PMI of theselected matrix for the original precoding by short term to the basestation.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is the block diagram of the wireless communication system usingclosed-loop spatial multiplexing according to one embodiment.

FIG. 2 is the diagram of the precoder according to the other embodiment.

FIG. 3 is the flowchart of the CL-MIMO according to other embodiment.

It will be appreciated that for simplicity and clarity of illustration,elements illustrated in the drawings have not necessarily been drawn toscale. For example, the dimensions of some of the elements areexaggerated relative to other elements for purposes of promoting andimproving clarity and understanding. Further, where consideredappropriate, reference numerals have been repeated among the drawings torepresent corresponding or analogous elements.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

Hereinafter, embodiments of the present invention will be described indetail with reference to the attached drawings.

There are a number of multi-antenna transmission schemes or transmissionsuch as transit diversity, closed-loop spatial multiplexing or open-loopspatial multiplexing. Closed-loop MIMO (CL-MIMO) relies on moreextensive feedback from the mobile terminal.

A unitary precoding is employed for Single User CL-MIMO (SU CL-MIMO),and unitary codebooks for different antenna configuration are defined.In LTE advance, it can be non-unitary also. Moreover, rank adaptation isalso considered in LTE to enhance the performance.

However, in LTE, there is no power allocation among different layers ifthe rank is larger than 1. It is well known that unitary precoding withwater filling power allocation is the optimal solution for CL-MIMO. Sothe original CL-MIMO in LTE is not optimal.

In this exemplary embodiment, a multi level precoding scheme is proposedfor CL-MIMO. In the proposed scheme, we consider the use of two levelprecoding. The first level precoding is for rank adaptation and powerallocation, and the second one is for unitary precoding. With theproposed scheme, we can get optimal solution for CL-MIMO. So it canincrease the CL-MIMO performance. By harmonizing rank adaptation andpower allocation, we can use fewer coding bits to show the sameinformation so that it can reduce the overhead. Moreover, by usingmultilevel precoding, we can separately feedback the PMI for each level.The first level PMI is feedbacked less frequently than the second one.So the feedback overhead is further reduced.

FIG. 1 is the block diagram of the wireless communication system usingclosed-loop spatial multiplexing according to one embodiment.

Referring to FIG. 1, the communication system may be any type ofwireless communication system, including but not limited to a MIMOsystem, SDMA system, CDMA system, OFDMA system, OFDM system, etc. In thecommunication system, the wireless communication system usingclosed-loop spatial multiplexing according to one embodiment comprises atransmitter 10 and a receiver 20. The transmitter 10 may act as a basestation, while the receiver 20 may act as a subscriber station, whichcan be virtually any type of wireless one-way or two-way communicationdevice such as a cellular telephone, wireless equipped computer system,and wireless personal digital assistant. Of course, thereceiver/subscriber station 20 can also transmits signals which arereceived by the transmitter/base station 10. The signals communicatedbetween the transmitter 10 and the receive 20 can include voice, data,electronic mail, video, and other data, voice, and video signals.

In operation, the transmitter 10 transmits a signal data stream throughone or more antennas and over a channel to a receiver 20, which combinesthe received signal from one or more receiver antennas to reconstructthe transmitted data. To transmit the signal, the transmitter 10prepares a transmission signal represented by the vector for the signal.

The transmitter 10 comprises a layer mapper 30 and a precoder 40.

The layer mapper 30 of the transmitter 10 maps one or two codewords,corresponding to one or two transports, to the layers N_(L) which mayrange from a minimum of one layer up to a maximum number of layers equalto the number of antenna ports. In case of multi-antenna transmission,there can be up to two transport blocks of dynamic size for each TTI(Transmission Time Interval), where each transport block corresponds toone codeword in case of downlink spatial multiplexing. In other words,the block of modulation symbols (one block per each transport block)refers to as a codeword. If there is only one codeword, we call itsingle codeword (SCW). Otherwise, we call it multiple codeword (MCW).

After layer mapping by the layer mapper 30, a set of N_(L) symbols (onesymbol from each layer) is linearly combined and mapped to the N_(A)antenna port by the precoder 40. This combining/mapping can be describedby means of a precoding matrix P of size N_(L)×N_(A).

In various example embodiments, the precoding matrix P is implementedwith the matrix P=WD, where D is a first level matrix for rankadaptation and power allocation, and W is a second level matrix for theoriginal precoding. It can be unitary or non-unitary without powerallocation information.

The precoder 40 has its own codebook, which is accessed to obtain atransmission profile and/or precoding information to be used to processthe input data signal to make best use of the existing channelconditions for individual receiver stations. In addition, the receive 20includes the same codebook for use in efficiently transferringinformation in either the feedback or feedforward channel, as describedherein below.

In various embodiments, the codebook is constructed as a compositeproduct codebook from separable sections, where the codebook index maybe used to access the different sections of the codebook. For example,one or more predetermined bits from the codebook index are allocated foraccessing the first level matrix, while a second set of predeterminedbits from the second level index is allocated to indicate the values forthe second level matrix.

In various embodiments, instead of having a single codebook at each ofthe transmitter 10 and the receiver 20, separate codebooks can be storedso that there is, for example, a codebook for the first level precodingmatrix W, a codebook for the second level matrix D. In such a case,separate indices may be generated wherein each index points to acodeword in its corresponding codebook, and each of these indices may betransmitted over a feedback channel to the transmitter, so that thetransmitter uses these indices to access the corresponding codewordsfrom the corresponding codebooks and determine a transmission profile orprecoding information.

FIG. 2 is the diagram of the precoder according to the other embodiment.

Referring to FIG. 2, after layer mapping by the layer mapper 30, a setof N_(L) symbols (one symbol from each layer) is linearly combined andmapped to the N_(A) antenna port by the precoder 40.

The precoder 40 comprises two level precoders 42 and 44 to optimize theperformance. The first level precoder 42 is for rank adaptation andpower allocation. The second level one 44 is for the original precoding.

In various example embodiments, the first precoder 42 may precode a setof symbols from the layer mapper 30 by means of a precoding matrix D ofsize N_(L)×N_(L). The second precoder 44 may also precode a set ofsymbols from the first precoder 42 by means of a precoding matrix W ofsize N_(L)×N_(A). The precoding matrix D is a first level matrix forrank adaptation and power allocation, and the precoding matrix W is asecond level matrix for the original precoding. As a result, the firstand the second precoder 42 and 44 precode a set of symbols by means ofthe matrix P=WD.

To assist the base station in selecting a suitable precoding matrix fortransmission by the transmitter (10), the receiver/mobile terminal 20may report channel information such as a recommended number of layers(expressed as a Rank Indication, RI) or a recommended precoding matrix(Precoding Matrix Index, PMI) corresponding to that number of layers,depending on estimates of the downlink channel conditions.

Referring to FIGS. 1 and 2 again, the receive 20 comprises a channelestimator 50 and a post-decoder 60.

The channel estimator 50 of the receive 20 estimates the downlinkchannel condition. The channel estimator 50 feedbacks at least one of RIand PMI to the transmitter 10. The channel estimator 50 may perform manykinds of codebook based PMI feedback.

The receive 20 estimates the channel by the channel estimator 50. Basedon the estimated channel information, then the receive 20 selects theprecoding matrix for each level from the corresponding codebooks, whichcan make the system have the highest sum rate. Once the precoding matrixfor each level is decided, the receiver/mobile terminal 20 separatelyfeedback the PMIs of both level to the transmitter 10.

There is codebook based PMI feedback where the receiver/mobile terminal20 feedbacks the precoding matrix index (PMI) of the favorite matrix inthe codebook to the transmitter/base station 10 to support CL-MIMO(closed MIMO) operation in wireless communication system.

The feedback frequency of the receive 20 is different for differentlevel precoding. The first level precoding is for rank adaptation andpower allocation, which is decided by the channel amplitude. The secondlevel precoding is for the original precoding, which is mainly decidedby the phase. Since the phase changes much faster than the amplitude,the change of PMI feedback for the first level precoding is also muchslower than the change of PMI feedback for the second one. So the firstlevel precoding is by long term feedback and the second one is by shortterm feedback. So multi level precoding can reduce the feedbackoverhead.

The transmitter 10 receives PMI feedback for the first level precodingby long term and PMI feedback for the second level precoding by shortterm. The transmitter 10 precodes the set of symbols by means of theprecoding matrix P=WD based on the two feedback PMIs as shown in FIG. 1,where D is the first level matrix for rank adaptation and powerallocation, and W is the second level matrix for the original precoding.In the other embodiment as shown in FIG. 2, the transmitter 10 precodesthe set of data symbols by means of the two level precoders 42 and 44based on the two feedback PMIs. For example, the first precoder 42 andthe second precoder 44 in turn precodes the set of data symbols by meansof each of matrices W or D based on the long and the short-term feedbackPMIs.

Then the transmitter 10 transmits the precoded data symbols by differentantennas.

The receive 20 recovers the original data symbols by post-decoder 60with the previous feedback precoding matrices combination. Thepost-decoder 60 processes the received signal and decodes the precodedsymbols.

FIG. 3 is the flowchart of the CL-MIMO according to other embodiment.

Referring to FIGS. 1 to 3, in the multilevel precoding CL-MIMO, thereceive 20 estimates the channel S10. Based on the estimated channelinformation, the receive 20 computes the sum rate for all the possiblecombinations of the two level precoding matrices from the codebooks.

The receive 20 picks one matrix from the first level codebook and picksanother one from the second level codebook. Then the receive 20 computesthe sum rate of the system when combining these two matrices togetherfor precoding. The receive 20 computes the sum rate for all the possiblecombinations and selects the one which has the highest sum rate. Inother words, the receive 20 selects the precoding matrix for each levelfrom the corresponding codebooks S20, which has the highest sum rateamong all the possible combinations.

Once the precoding matrix for each level is decided, the receive 20feedbacks the PMIs of the matrix in the best combination to thetransmitter 10.

In multiple level precoding, the first level is for rank adaptation andpower allocation, which is decided by the channel amplitude. The secondlevel precoding is the original, which is mainly decided by the phase.Since the phase changes much faster than the amplitude, the change PMIfeedback of for the first level precoding is also much slower than thesecond one.

In multilevel precoding, the first level is for rank adaptation andpower allocation. So the codebook is not unitary. By harmonizing rankadaptation and power allocation, we can use fewer coding bits to showthe same information so that it can reduce the overhead.

1) For rank adaptation information in the codebook, the identity matrixwith some 1 element replaced by zeros can be used. It can also indicatethe information that equal power allocation is used. So that we reducethe coding bits

2) For power allocation, it attach to each rank. The total power is notchanged. The diagonal matrix is used in the codebook for powerallocation.

3) For the exact value for power allocation, we can drive them fromtransmit adaptive antennas (TxAA) codebook, or by simulation to get theoptimized value.

Table 1 gives an example of the first level codebook for 2×2 CL MIMO.1.

TABLE 1 Codebook index codebook Meaning 0 $\quad\begin{bmatrix}1 & 0 \\0 & 0\end{bmatrix}$ Rank 1 1 $\frac{1}{\sqrt{2}}\begin{bmatrix}1 & 0 \\0 & 1\end{bmatrix}$ Rank 2 Equal power allocation 2 $\quad\begin{bmatrix}\frac{2}{\sqrt{5}} & 0 \\0 & \frac{1}{\sqrt{5}}\end{bmatrix}$ Rank 2 None Equal power allocation 3$\quad\begin{bmatrix}\frac{1}{\sqrt{5}} & 0 \\0 & \frac{2}{\sqrt{5}}\end{bmatrix}$ Rank 2 None Equal power allocation

Referring to the table 1, each of codebook indices has each ofcodebooks. Each of codebooks means rank adaptation and power allocation.For example, the codebook of codebook index “0” is

$\begin{bmatrix}1 & O \\O & O\end{bmatrix},$

which means rank 1. The codebook of codebook index “1” is

${\frac{1}{\sqrt{2}}\begin{bmatrix}1 & 0 \\0 & 1\end{bmatrix}},$

which means rank 2 and equal power allocation. The codebook of codebookindex “2” is

$\begin{bmatrix}\frac{2}{\sqrt{5}} & 0 \\0 & \frac{1}{\sqrt{5}}\end{bmatrix},$

which means rank 2 and unequal power allocation. The codebook ofcodebook index “3” is

$\begin{bmatrix}\frac{1}{\sqrt{5}} & o \\o & \frac{2}{\sqrt{5}}\end{bmatrix},$

which means rank 2 and unequal power allocation.

So the receive 20 does not need feedback the PMIs for both levels allthe time. In every feedback, the receive 20 feedbacks the PMI for thesecond level precoding. For every several feedbacks, the receive 20feedbacks the PMI for the first level precoding.

Before the PMI feedbacks, the receive 20 checks whether the PMI of thefirst level need to be feedbacked S30 because the feedback of the firstlevel precoding is less frequent than the feedback of the second one. Ifthe first level precoding is needed, the receive 20 feedbacks the PMIsfor both level precodings to the transmitter separately S40. Otherwise,it only feedbacks the second level precoding S50.

At the transmitter 10, it precodes the data symbols by using the twolevel precoder based on the feedback PMIs, and then transmit theprecoded data symbols by different antennas.

At the receiver 20, it recovers the original data symbols bypost-decoder 60 with the previous feedback precoding matricescombination.

In the original LTE CL-MIMO, there is no power allocation amongdifferent layers. It is well known that unitary precoding with waterfilling power allocation is the optimal solution for CL-MIMO. So theoriginal CL-MIMO in LTE is not optimal.

In this exemplary embodiment, a multi level precoding scheme is proposedfor CL-MIMO. In the proposed scheme, we consider use two levelprecoding. The first level precoding is for rank adaptation and powerallocation, and the second one is for original precoding. With theproposed scheme, we can get optimal solution for CL-MIMO. So it canincrease the CL-MIMO performance.

By harmonizing rank adaptation and power allocation, we can use fewercoding bits to show the same information so that it can reduce theoverhead.

By using multilevel precoding, we can separately feedback the PMI foreach level. Since the change in the feedback PMI for the first levelprecoding is much slower than that of the second one, the feedbackfrequency is different for different level precoding. We feed back thefirst level PMI less frequently than the second one. So multilevelprecoding can reduce the feedback.

The methods and systems as shown and described herein may be implementedin software stored on a computer-readable medium and executed as acomputer program on a general purpose or special purpose computer toperform certain tasks. For a hardware implementation, the elements usedto perform various signal processing steps at the transmitter (e.g.,coding and modulating the data, precoding the modulated signals,preconditioning the precoded signals, and so on) and/or at the receiver(e.g., recovering the transmitted signals, demodulating and decoding therecovered signals, and so on) may be implemented within one or moreapplication specific integrated circuits (ASICs), digital signalprocessors (DSPs), digital signal processing devices (DSPDs),programmable logic devices (PLDs), field programmable gate arrays(FPGAs), processors, controllers, micro-controllers, microprocessors,other electronic units designed to perform the functions describedherein, or a combination thereof. In addition or in the alternative, asoftware implementation may be used, whereby some or all of the signalprocessing steps at each of the transmitter and receiver may beimplemented with modules (e.g., procedures, functions, and so on) thatperform the functions described herein. It will be appreciated that theseparation of functionality into modules is for illustrative purposes,and alternative embodiments may merge the functionality of multiplesoftware modules into a single module or may impose an alternatedecomposition of functionality of modules. In any softwareimplementation, the software code may be executed by a processor orcontroller, with the code and any underlying or processed data beingstored in any machine-readable or computer-readable storage medium, suchas an on-board or external memory unit.

Although the described exemplary embodiments disclosed herein aredirected to various MIMO precoding systems and methods for using same,the present invention is not necessarily limited to the exampleembodiments illustrate herein. For example, various embodiments of aMIMO precoding system and design methodology disclosed herein may beimplemented in connection with various proprietary or wirelesscommunication standards, such as IEEE 802.16e, 3GPP-LTE, DVB and othermulti-user MIMO systems. Thus, the particular embodiments disclosedabove are illustrative only and should not be taken as limitations uponthe present invention, as the invention may be modified and practiced indifferent but equivalent manners apparent to those skilled in the arthaving the benefit of the teachings herein. Accordingly, the foregoingdescription is not intended to limit the invention to the particularform set forth, but on the contrary, is intended to cover suchalternatives, modifications and equivalents as may be included withinthe spirit and scope of the invention as defined by the appended claimsso that those skilled in the art should understand that they can makevarious changes, substitutions and alterations without departing fromthe spirit and scope of the invention in its broadest form.

Benefits, other advantages, and solutions to problems have beendescribed above with regard to specific embodiments. However, thebenefits, advantages, solutions to problems, and any element(s) that maycause any benefit, advantage, or solution to occur or become morepronounced are not to be construed as a critical, required, or essentialfeature or element of any or all the claims. As used herein, the terms“comprises,” “comprising,” or any other variation thereof, are intendedto cover a non-exclusive inclusion, such that a process, method,article, or apparatus that comprises a list of elements does not includeonly those elements but may include other elements not expressly listedor inherent to such process, method, article, or apparatus.

1. A communication method comprising: estimating a downlink channelbased on a signal received from a base station; transmitting afirst-type precoding matrix index (PMI) to the base station;transmitting a second-type PMI to the base station; and receiving adownlink signal from the base station, wherein transmitting thefirst-type PMI is periodically performed based on a first period, andtransmitting the second-type PMI is periodically performed based on asecond period.
 2. The method of claim 1, wherein the first period isshorter than the second period.
 3. The method of claim 1, wherein thesecond period is a multiple of the first period.
 4. The method of claim3, further comprising decoding the received -downlink signal based onthe first-type PMI and the second-type PMI.
 5. The method of claim 1,further comprising decoding the received downlink signal based on thefirst-type PMI and the second-type PMI.
 6. A mobile terminal comprising:a processor; and a memory operably coupled to the processor, wherein theprocessor, when executing program instructions stored in the memory, isconfigured to: cause the mobile terminal to estimate a downlink channelbased on a signal received from a base station; cause the mobileterminal to transmit a first-type precoding matrix index (PMI) to thebase station; cause the mobile terminal to transmit a second-type PMI tothe base station; and cause the mobile terminal to receive a downlinksignal from the base station, wherein the processor is furtherconfigured to cause the mobile terminal to transmit the first-type PMIperiodically based on a first period, and cause the mobile terminal totransmit the second-type PMI periodically based on a second period. 7.The mobile terminal of claim 6, wherein the first period is shorter thanthe second period.
 8. The mobile terminal of claim 6, wherein the secondperiod is a multiple of the first period.
 9. The mobile terminal ofclaim 8, wherein the processor is further configured to decode thereceived downlink signal based on the first-type PMI and the second-typePMI.
 10. The mobile terminal of claim 1, wherein the processor isfurther configured to decode the received downlink signal based on thefirst-type PMI and the second-type PMI.
 11. A device for a mobileterminal, the device comprising: a processor; and a memory operablycoupled to the processor, wherein the processor, when executing programinstructions stored in the memory, is configured to: cause the mobileterminal to estimate a downlink channel based on a signal received froma base station; cause the mobile terminal to transmit a first-typeprecoding matrix index (PMI) to the base station; cause the mobileterminal to transmit a second-type PMI to the base station; and causethe mobile terminal to receive a downlink signal from the base station,wherein the processor is further configured to cause the mobile terminalto transmit the first-type PMI periodically based on a first period, andcause the mobile terminal to transmit the second-type PMI periodicallybased on a second period.
 12. The device of claim 11, wherein the firstperiod is shorter than the second period.
 13. The device of claim 12,wherein the second period is a multiple of the first period.
 14. Thedevice of claim 13, wherein the processor is further configured todecode the received downlink signal based on the first-type PMI and thesecond-type PMI.
 15. The device of claim 11, wherein the processor isfurther configured to decode the received downlink signal based on thefirst-type PMI and the second-type PMI.